1. Field of the Invention
The present invention relates, in general, to a printed circuit board with a weak magnetic field sensor and a method of fabricating the same and, more particularly, to a printed circuit board with a weak magnetic field sensor, in which a soft magnetic core, an excitation circuit, and a detection circuit lie at right angles to each other on upper and lower sides of the printed circuit board to sense a weak-magnetic field having a similar strength to the earth's magnetic field, and a method of fabricating the same.
2. Description of the Prior Art
The recent trend of expansion of various kinds of additional information services associated with the spread of mobile phones and mobile terminals brings about adoption of location information services as basic functions, and demands for more detailed and convenient services are growing.
A sensor capable of precisely sensing a user's location is needed to check location information, and a weak magnetic field sensor, which is used as a unit for providing the location information as shown in FIG. 1, senses the earth's magnetic field to detect the user's location.
FIG. 1 schematically illustrates a conventional weak magnetic field sensor, FIG. 2a is a timing diagram of a magnetic field generated from a first magnetic core, FIG. 2b is a timing diagram of a magnetic field generated from a second magnetic core, FIG. 2c is a timing diagram of magnetic flux density at the first magnetic core, FIG. 2d is a timing diagram of magnetic flux density at the second magnetic core, and FIGS. 2e and 2f are timing diagrams of first and second induced voltages (Vind1, Vind2) induced to a detection coil, and the total of the first and second induced voltages (Vind1+Vind2), respectively.
As shown in FIG. 1, the conventional weak magnetic field sensor includes first and second magnetic cores 1a, 1b having a shape of a large bar, excitation coils 2a, 2b, which are wound around the first and second magnetic cores 1a, 1b in a predetermined direction at regular intervals so that the wound excitation coils 2a, 2b form a figure—8 pattern to excite a magnetic field, and detection coils 3a, 3b, which are wound around the first and second magnetic cores 1a, 1b in a predetermined direction at regular intervals to detect the magnetic field generated from the first and second magnetic cores 1a, 1b. 
Referring to FIGS. 2a to 2f, in which operation of the conventional weak magnetic field sensor is illustrated, an internal magnetic field (H1) of the first magnetic core 1a caused by an excitation current of an alternating current is expressed by ‘Hext (external magnetic field)+Hexc (magnetic field caused by the excitation coils)’, and an internal magnetic field (H2) of the second magnetic core 1b is expressed by ‘Hext-Hexc’.
Additionally, a magnetic flux density (B1) of the first magnetic core 1a is expressed by ‘Bext (magnetic flux density by the external magnetic field)+Bexc (magnetic flux density by the excitation coils)’, and a magnetic flux density (B2) of the second magnetic core 1b is expressed by ‘Bext-Bexc’.
In other words, the internal magnetic fields (H1, H2) of the first and second magnetic cores 1a, 1b are generated in a contrary direction each other, and so are the magnetic flux densities (B1, B2).
At this time, the first and second induced voltages (Vind1, Vind2), generated from the first and second magnetic cores 1a, 1b and detected by the detection coils 3a, 3b, are shown in FIG. 2e. 
Since the winding of the detection coils 3a, 3b is conducted in such a way that variations of magnetic fluxes generated from the first and second magnetic cores 1a, 1b are summed, a voltage, detected by the detection coils 3a, 3b, is measured by offsetting the first and second induced voltages (Vind1, Vind2) against each other as shown in FIG. 2f. 
That is to say, the magnetic fields (Hexc), caused by the excitation coils, are axially applied to the first and second magnetic cores 1a, 1b in a direction counter to each other to be offset into 0, and the external magnetic fields (Hext) are axially applied to the first and second magnetic cores 1a, 1b in the same direction, thereby the offset of the external magnetic fields is avoided.
Accordingly, an amplitude of the external magnetic field (Hext) may be obtained by measuring an amplitude of the total of the first and second induced voltages (Vind1+Vind2).
However, the conventional weak magnetic field sensor is problematic in that it is difficult to maintain a location accuracy when the excitation coils 2a, 2b and detection coils 3a, 3b are wound around the magnetic cores 1a, 1b, and that since the coils are vulnerable to temperatures, light, and surface materials for detection, precisions of characteristic values are poor.
Further, the conventional weak magnetic field sensor has disadvantages in that since the excitation coils 2a, 2b and detection coils 3a, 3b are directly wound around the magnetic cores 1a, 1b, the coils are frequently snapped.
Another disadvantage of the conventional weak magnetic field sensor is that since its size and its power consumption are large, it cannot follow the trend of miniaturization and power reduction of electronic products.
To avoid the disadvantages of the conventional weak magnetic field sensor, U.S. Pat. Nos. 5,936,403 and 6,270,686 disclose a weak magnetic field sensor. In these patents, an amorphous core is formed by stacking amorphous boards, each having a circular conductor pattern, on opposite sides of an epoxy base board, which has a particular pattern etched thereon and the capacity for vertical conductivity, and epoxy bases, which have a coil X and a coil Y, respectively, are stacked on the top and bottom surfaces of the amorphous core.
However, the U.S. Pat. Nos. 5,936,403 and 6,270,686 are problematic in that since the epoxy base board is etched to have the circular conductor pattern, the stacking is conducted in such a way that etched portions correspond in position to each other to produce the amorphous core, and the epoxy base board, having the coils X and Y thereon, is stacked on upper and lower sides of the amorphous core, the fabricating process of the weak magnetic field sensor is complicated and its production costs are increased because the sensor has many layers.
Furthermore, U.S. Pat. Nos. 5,936,403 and 6,270,686 are disadvantageous in that since lands of the coils are clustered together inside the circular amorphous core, the number of coil windings is limited, thus reducing sensitivity in detecting a weak-magnetic field due to low coil density per unit length, thereby running counter to the trend of miniaturization of electronic products.
Korean Pat. Laid-Open Publication No. 2004-11287, which was submitted on Jul. 30, 2002 by the applicant of the present invention, suggests a printed circuit board, having biaxial magnetic field detecting elements integrated thereon, to avoid the above disadvantages. The printed circuit board comprises a first soft magnetic core having a shape of two bars or rectangular rings extended in a first axial direction; a first excitation coil wound around the first soft magnetic core, resulting in formation of a metal film; a first detection coil wound around the first soft magnetic core in such a way that the first detection coil and first excitation coil are alternatively positioned on the same plane, causing formation of a metal film; a second soft magnetic core with a shape of two bars or rectangular rings extended in a second axial direction at right angles to the first soft magnetic core; a second excitation coil wound around the second soft magnetic core, resulting in formation of a metal film; and a second detection coil wound around the first soft magnetic core in such a way that the second detection coil and second excitation coil are alternatively positioned on the same plane, leading to formation of a metal film.
However, Korean Pat. Laid-Open Publication No. 2004-11287 has disadvantages in that an interface between the soft magnetic core and an insulating layer (e.g. epoxy resin layer) is in good order after fabrication of the printed circuit board, including the biaxial magnetic field detecting elements integrated thereon, is completed, but after the printed circuit board is tested to estimate its life, delamination occurs at the interface between the soft magnetic core and insulating layer.
Additionally, Korean Pat. Laid-Open Publication No. 2004-11287 is problematic in that since stripping occurs at interfaces between a developed dry film and the surface of the soft magnetic core during the lithography process for forming the soft magnetic core and the circuit pattern, an etching solution penetrates between the dry film and the surface of the soft magnetic core during an etching process, and thus, there exists an etching deviation between the soft magnetic core and the circuit pattern (i.e. copper foil).